1. Field
Embodiments of the present invention relates to photonic devices and, more particularly, to stabilization and control of in-fiber photonic devices.
2. Background Information
An optical transmission system transmits information from one place to another by way of a carrier whose frequency is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, or a lightwave signal.
An optical transmission system typically includes several optical fibers. Each optical fiber includes several channels. A channel is a specified frequency band of an electromagnetic signal, and is sometimes referred to as a wavelength. One link of an optical transmission system typically has a transmitter, the optical fiber, and a receiver. The transmitter converts an electrical signal into the optical signal and launches it into the optical fiber. The optical fiber transports the optical signal to the receiver. The receiver converts the optical signal back into an electrical signal.
An optical transmission system that transmits more than one channel over the same optical fiber is sometimes referred to as a multiple channel system. The purpose for using multiple channels in the same optical fiber is to take advantage of the unprecedented capacity offered by optical fibers. Essentially, each channel has its own wavelength, and all wavelengths are separated enough to prevent overlap.
One way to transmit multiple channels is through wavelength division multiplexing, whereupon several wavelengths are transmitted in the same optical fiber. Typically, several channels are interleaved by a multiplexer, launched into the optical fiber, and separated by a demultiplexer at a receiver. Along the way, channels may be added or dropped using an add/drop multiplexer or switched using optical cross-connect switches. Wavelength division demultiplexing elements separate the individual wavelengths using frequency-selective components, which can provide high reflectivity and high wavelength selectivity with the aim of increasing the transmission capacity of optical fibers.
Many of these frequency-selective components are in-fiber photonic devices in that the devices are part of an optical fiber. In-fiber devices have a large number of advantages. One advantage is that coupling of optical signals in and out of the optical fiber to another discrete photonic device (e.g., discrete filter) is avoided, which allows the optical transmission system to achieve much lower insertion losses and to increase long-term device reliability. An additional advantage is that polarization effects are reduced because cylindrical symmetry is maintained.
One of the limitations of in-fiber devices is that they are difficult to control using external inputs, to tune or to stabilize device properties, for example. For instance, one such in-fiber device is a fiber Bragg grating, which can be used as a temporally invariant optical filter. The physical properties (e.g., strain, temperature) of fiber Bragg gratings typically should be stabilized so that the filtering properties of the gratings are stabilized. When a fiber Bragg grating is attached to a substrate, however, the filtering properties of the fiber Bragg grating may be affected by the physical characteristics of the substrate.
Morey et al., U.S. Pat. No. 5,042,898 (hereinafter “Morey”), disclose temperature compensated embedded Bragg grating filters in which temperature-varying longitudinal strains are configured to compensate central wavelength changes attributable to temperature changes. Morey is limited, however, in that it does not compensate for other environmental conditions to which embedded Bragg grating filters may be subjected.